Indirectly heated cathode



DeC. 21, 1965 J. w. HoLLlNswoRTH ETAL 3,225,246

INDIREGTLY HEATED CATHODE Filed March 27, 1963 United States Patent O 3,225,246 INDIRECTLY HEATED CATHODE James William Hollingsworth and Murray Robert Horton, Lancaster, Pa., assignors to Radio Corporation f America, a corporation of Delaware Filed Mar. 27, 1963, Ser. No. 268,387 Claims. (Cl. 313-270) This invention relates to indirectly-heated cathodes for electron tubes, and particularly to an improved heater structure therefor.

Indirectly heated cathodes usually include a metal sleeve having electron-emitting oxides on the outer surface thereof. A heater in the form of a wire, made of tungsten for example, is disposed insulatingly within the sleeve in heat transfer relation thereto. The wire is connected to a suitable current source for heating the Wire by current losses therein. The heated wire transfers heat produced therein to the sleeve for heating the oxides on the sleeve to emitting temperature.

Some types of tubes are involved in applications in which the heater Wire is subjected to a relatively high voltage. Such high voltage heater wire has relatively small cross sectional dimensions. For example, while a six volt tungsten heater wire may have a diameter of from eight to ten mils, a twenty-six volt tungsten heater wire on the other hand has a diameter of from two to lve mils for satisfactory operation.

Several problems are associated with high voltage heaters. One of these problems arises as a consequence of the fact that the wires of such heaters must be relatively thin and therefore lack tensile strength. Such relatively thin heater Wires are easily ruptured and extreme care must be observed in their handling. Another problem relates to the electrical insulation required by virtue of the high voltage used. Such insulation must be adequate to electrically isolate the heater from the cathode sleeve, as well as to insulated adjacent turns from each other where the heater is in the form of a closely-wound coil. A further problem involves the support of the heater wire. Such support should be relatively rugged so as to avoid displacements of the heater wire from an initially fixed position thereof. And finally, it is desirable that the heater comprise a structure contributing to facility in assembling the heater within a cathode sleeve.

Accordingly, it is an object of the invention to provide an improved indirectly-heated cathode having a relatively high voltage heater structure which avoids the foregoing problems.

Another object is to provide a high voltage heater structure for an indirectly-heated cathode wherein a relatively thin heater wire is ruggedly supported and electrically insulated from adjacent metallic elements.

A further object is to provide a heater structure contributing to facility in assembling the heater in a cathode sleeve and xing the structure in desired position within the sleeve.

In one example 0f the invention, a heater structure may comprise a ceramic tube having a closely wound coil of insulated tungsten wire thereon. The ceramic tube is fixed at its ends to two spaced metallic collars or flanges, to which collars the opposite ends of the heater wire are connected, as being embedded in metalized coatings on the end surfaces of the tube. One of the collars which may be called the upper collar, is snugly telescoped over a conducting rod and fixed thereto. The other or lower collar has a central opening sufficiently large to provide clearance between the collar and rod. The lower collar has a larger outer diameter than the upper collar outer diameter to provide a snug engagement for welding between the outer periphery of the lower collar and the inner surface of a cathode sleeve. Prior to insertion of the heater struc- ICC ture within the cathode sleeve, a ceramic disc having a central opening is telescoped over and into engagement with an end portion of the conducting rod extending through the upper collar. The ceramic disc has a diameter substantially equal to that of the lower collar for snugly engaging the inner wall of the cathode sleeve.

This provides a highly rugged cathode-heater sub-assembly for an electron tube, permitting the use of relatively thin heater wire for high voltage heater energization.

Further objects and features of the invention will become apparent as the present description continues.

Referring to the drawing for a consideration of an exemplary embodiment of the invention,

FIG. 1 shows a longitudinal cross-section of a cathodeheater structure for an electron tube; and

FIG. 2 is a fragmentary view taken along the line 2-2 of FIG. l.

The cathode-heater structure shown in FIG. 1 comprises a tubular cathode 10 including an active portion 12 having thereon a coating 14 of electron emitting material which may be in the form of carbonates of barium, strontium and calcium. The cathode may be made of a material such as nickel. During subsequent processing of the cathode after incorporation in an electron tube, the carbonates referred to are converted to oxides. The cathode 10 also includes a ange 16 which may serve as a cathode lead-in in the completed tube. A thinned-down portion 18 of the cathode serves as an effective heat dam to reduce heat losses from the active portion 12 to the flange 16. The upper end of cathode 10 as viewed in FIG. l is partly closed by an annular inturned end portion 20 dening an opening 22.

Within the cathode 10 is disposed a heater assembly 24. This assembly comprises a cylinder 26 made of a ceramic such as Forsterite having thereon a closely Wound coil 23 of tungsten wire 30 coated with insulating material 32 such as aluminum oxide, to insulate adjacent turns of the coil from each other.

To the upper end of the cylinder 26, as viewed in FIG. l, is fixed a metal collar 34, which may be made of nickel. The collar 34 as well as the coil 28 have an appreciably smaller diameter than the inner diameter of the active portion 12 of the cathode, so as to be space insulated therefrom. For fixing the collar 34 to the upper end surface of the cylinder 26, such upper end surface is suitably metalized as by the use of conventional molybdenum salts, and the resultant molybdenum coating is plated with copper to permit brazing of such surface to an annular portion of the collar 34. The collar 34 is provided with a central opening for snugly embracing a metal rod or pin 36, made of nickel, for example.

For fixing the upper end of the wire 30 of the heater coil 28 an uncoated portion 54 of the wire (FIG. 2) is clamped between the upper collar 34 and 'the upper surface of the cylinder 26, As shown in FIG. 1, when the upper collar 34 is brazed to the metalized coating 56, at a temperature of about 1140 C., the uncoated portion 54 of the heater wire is embedded in the softened copper of the metalized coating and in good electrical Contact with the metal collar 34. Since the upper collar 34 conductively engages the rod 36 and the rod in turn is conductively xed to ange 48, the latter ange may serve effectively as a lead-in for the upper end of the heater coil 28.

The lower end portion (not shown) of the coil 28 is also free of insulation and is clamped between the metalized lower end surface of the cylinder 26, and an annular portion of a lower nickel collar 38 engaged by the metalized lower end surface of the cylinder. When the copper of the metalized coating is softened at about l C. in a brazing operation, the bare lower end portion of the Wire of coil 2S is embedded in the metal joint to the lower collar 38. Since this collar is fixed to the inner surface of the cathode as the aforementioned, the flange 16 of the cathode may serve conveniently as a lead-in for the lower end of the heater wire of coil 28. The engagement of the lower end of the heater wire with the collar 38 may be similar to the embedded engagement described before herein in connection with the upper end of the heater wire.

The collar 38 defines a central opening 40 having a diameter for providing an appreciable space clearance with respect to rod 36. The outer diameter of lower collar 38 is larger than that of the upper collar 34, for snug engagement with the inner wall of the lower end portion of the active portion 12 of the cathode. The collar 38 is fixed to the inner wall of the active portion 12 as by an annular weld 42.

For insulating the upper end portion of the heater structure 24 from the cathode 10, an insulating disc 44 made of a ceramic such as aluminum oxide, is provided with a central opening for receiving the upper end portion of rod 36 in a sliding engagement. The outer diameter of the disc 44 is larger than the outer diameter of the upper collar 34, and snugly engages the inner wall of cathode 10. Prior to fixing the lower collar 38 to the inner surface of the cathode, the heater assembly is urged upwardly in the cathode until the upper surface of the disc 44 abuts against the inner surface of the inturned portion 20 of the cathode. The opening 22 in the upper end of the cathode is sufficiently large to permit the upper end of the rod 36 to extend therethrough without danger of electrical shorts therebetween.

For providing a heater assembly that lends itself to convenient handling during insertion thereof into the cathode 10, the rod 36 is fixed to the upper collar 34 by means of a metal ring or eyelet 46 which may be fixed as by welding to both the collar 34 and the rod 36. An insulating ring 5f) made of a ceramic such as aluminum oxide, for example, and a metal washer 52 made of an alloy such as Kovar, are positioned within a flange 16 and between the transverse portions of flange 16 and flange 48. The washer 52 provides a joint of graded expansion characteristics between the insulating ring 50 and the flange 16. To facilitate the fixing of opposite faces of the ring 50 to the fiange 48 and washer 52, such opposite faces are suitably metalized in a manner which may involve the use of conventional molybdenum salts as aforementioned. The periphery of washer S0 is not metalized so that the faces are insulated from each other.

In assembling the heater structure 24 in the cathode t 10, the heater structure may be held by the rod 36 in position so that the collar 34 thereof is uppermost. Thereafter, the insulating disc 44 may be threaded over the upper end of the rod 36 and in seated relation with respect to collar 34. The cathode may then be telescoped downwardly over the heater structure until the disc 44 abuts against the inner surface of the inturned portion of the cathode. When in this position, the lower collar 38 may be fixed to the inner surface of the cathode. It will be noted that the lower collar 38 is fixed to the active and relatively thick portion 12 of the cathode. This contributes to ruggedness of the assembly.

Thereafter, the washer 52 and insulating ring 50 are positioned on the transverse portion of flange 16, with the assembly preferably supported in such a way as to cause the flange 16 to extend upwardly. Thereafter, the flange 48 is threaded over the adjacent end of the rod 36 and is seated on a metalized surface of ring 50. With suitable brazing material, such as BT solder interposed between the rod 36 and flange 48, between the opposite surfaces of ring 50 and the flange 48 and washer 52, and between this washer and flange 16, the assembly is heated in a reducing atmosphere such as hydrogen in an oven at a temperature of about 1l40 C. for about ten minutes for brazing together the parts referred to.

Thereafter the emitting carbonates, held in a suitable binder, are applied to the active portion 12 of the cathode, as by spraying, to provide the coating 14. The application of the emitting coating 14 after the oven brazing step aforementioned has been carried out, protects the coating from harm by the oven temperature used.

The cathode-heater structure described provides a rugged support for the heater coil 28, and facilitates the operation of assembling the heater structure within the cathode. The parts conveniently may be made to such close tolerance as to render self-jigging feasible.

The tungsten wire employed in the coil of the heater structure may have a thickness of from 2 to 4 mils, for service with a heater supply of 26 volts. The insulating coating 32 on the tungsten wire may be made of a material such as aluminum oxide and having a thickness of about 2 mils. Assurance of continuous electrical contact between the ends of the wire of the coil and the collars 34, 38 is provided by the novel imbedment of the bare ends of the coil in the metalized coatings on the end surfaces of the cylinder 26.

What is claimed is:

1. A cathode-heater structure for an electron tube comprising:

(a) a tubular cathode,

(b) a heater assembly within said cathode,

(l) said assembly including a conducting rod coaxial with said cathode,

(2) means including a metal collar conductively engaging said rod and supporting said heater on said rod, said collar having opposite end faces and a smaller axial dimension than lateral dimension, and

(3) an insulating disc seated on said one of said end faces of said collar and having an opening snugly receiving said rod, said disc extending transversely of said cathode and snugly engaging the internal surface of said cathode,

(4) said collar having a smaller outer diameter than said disc whereby said disc electrically insulates said collar from said cathode.

2. A cathode-heater sub-assembly for an electron tube comprising:

(a) a tubular cathode (b) a heater structure within said cathode, said heater structure comprising (l) an insulating cylinder having a coil of heater wire thereon,

(2) a rst metal collar fixed to one end of said cylinder with a free end portion of said wire clamped between said first collar and said one end of the cylinder, said first collar being conductively fixed to the inner surface of said cathode,

(3) a second metal collar fixed to the other end of said cylinder, with the other free end portion of said wire clamped between said second collar and said cylinder, said second metal collar having an opening therethrough and an outer diameter smaller than the inner diameter of said cathode,

(4) a conducting rod extending through said opening in said second collar and conductively fixed thereto, and

(5) an insulating disc having a central opening snugly receiving a portion of said rod adjacent to said second collar, said disc having an outer diameter for snugly engaging the inner wall of said cathode for electrically insulating said second collar from said cathode.

3. A cathode-heater sub-assembly for an electron tube comprising:

(a) a tubular cathode having a relatively thin and a relatively thick portion,

(b) a heater structure within said cathode, said heater structure comprising (1) an insulating cylinder having a coil of heater wire thereon,

(2) a first metal collar fixed to one end of said cylinder with a free end portion of said wire clamped between said first collar and said one end of the cylinder, said first collar being conductively fixed to the inner surface of said relatively thick portion of said cathode, and having an opening,

(3) a second metal collar iixed to the other end of said cylinder, with the other free end portion of said wire clamped between said second collar and said cylinder end, said second metal collar having an opening and being space insulated from said cathode,

(4) a conducting rod extending through said openings in said first and second collars and conductively fixed to said second collar only, and

(5) an insulating disc having a central opening snugly receiving a portion of said rod adjacent to said second collar, said disc having an outer diameter for snugly engaging the inner wall of said cathode for electrically insulating said second collar from said cathode,

(6) whereby said rod is adapted to lead electrical energy to one end of said wire and said cathode is adapted to lead electrical energy to the other end of said Wire.

4. An electron tube subassembly comprising an indirectly heated cathode having a heater structure cornprising:

(a) an insulating support,

(b) a heater wire on said support,

(c) a lead-in conductor,

(d) and means comprising a metal collar conductively connecting one end of said wire to said lead-in conductor, said one end being clamped between said collar and a surface of said support for making electrical connection of said wire to said collar.

5. An electron tube subassembly comprising an indirectly-heated cathode having a heater structure comprising:

(a) an insulating support having a metalized surface,

(b) a heater Wire on a portion of said support spaced from said surface,

(c) a lead-in conductor,

(d) and means comprising a metal collar conductively connecting one end of said wire to said lead-in conductor, said one end being clamped between said collar and said metalized surface of said support.

6. An electron tube subassembly comprising an indirectly heated tubular cathode having a heater structure comprising:

(a) an insulating cylinder,

(b) a `coil of heater wire wound on said cylinder,

(c) a first metal collar fixed to one end of said cylinder and clamping an end of said wire between said one end and said iirst metal collar, and

(d) a second metal collar xed to the other end of said cylinder and clamping between said other end and said second collar and the other end of said wire.

7. An electron tube subassembly comprising an indidirectly heated cathode, said cathode having a heater structure comprising:

(a) an insulating cylinder having metalized end surfaces,

(b) a coil of heater Wire wound on the outer surface of said cylinder,

(c) a first metal collar fixed to one metalized end surface of said cylinder and clamping an end of said wire between said one end surface and said first metal collar for imbedding said end in said metalized surface, and

(d) a second metal collar fixed to the other rnetalized end surface of said cylinder, the other end of said wire being imbedded in the metal of said other metalized end surface.

8. An electron tube subassembly comprising an indirectly heated cathode having a heater structure comprising:

(a) an insulating support having two spaced metalized portions,

(b) a heater wire having two ends and mounted on said support between said portions,

(c) two lead-ins for said assembly,

(d) means including one of said portions of said insulating support fixing one end of said heater wire to one of said lead-ins, and

(e) means including the other of said portions of said insulating support fixing the other end of said heater wire to the other of said lead-ins.

9. A heater structure for an indirectly-heated tubular cathode, comprising:

(a) an insulating cylinder,

(b) heater wire supported on said cylinder,

(c) two metal collars each engaging an end of said cylinder, one of said collars having a larger outer diameter than the other for a snug fit in the tubular cathode,

(d) a conducting rod fixed to the smaller of said collars and space insulated from the larger of said collars, and

(e) an insulating disc having an opening snugly receiving a portion fo said rod adjacent to said smaller collar, said disc having outer diameter substantially equal to that of said larger collars for a snug fit in said cathode.

10. A heater structure for an indirectly-heated tubular cathode, comprising:

(a) an insulative cylinder having metal coatings on end surfaces thereof,

(b) heater Wire supported on and between the ends of said cylinder and having ends imbedded in said coatings,

(c) two metal collars each fixed to a different metal coating on the end of said cylinder, one of said collars having a larger outer diameter than the other for a snug fit in the tubular cathode,

(d) a conducting rod fixed to the smaller of said collars and space insulated from the larger of said collars and (e) an insulating disc having an opening snugly receiving a portion of said rod adjacent to said smaller collar, said disc having an outer diameter substantially equal to that of said larger collar for a snug fit in said cathode, and for insulating said smaller collar from said cathode.

References Cited by the Examiner UNITED STATES PATENTS 1,790,153 1/1931 Hull 313-337 X 1,941,074 12/1933 Bond et al 313-270 X 2,510,639 6/1950 Koch 313-340 X 2,842,701 7/1958 Cronin 313-340 JOI-IN W. HUCKERT, Primary Examiner. DAVID J. GALVIN, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTlON Patent No. 3,225,246 December 2l, 1965 James William Hollingsworth et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 36, for "fo" read of line 37, after "having" insert an line 38, for "collars" read collar same column 6, line 48, for "end" read ends Signed and sealed this 20th day of December 1966.

(SEAL) Attest:

Commissioner of Patents Attesting Officer 

1. A CATHODE-HEATER STRUCTURE FOR AN ELECTRON TUBE COMPRISING: (A) A TUBULAR CATHODE, (B) A HEATER ASSEMBLY WITHIN SAID CATHODE, (1) SAID ASSEMBLY INCLUDING A CONDUCTING ROD COAXIAL WITH SAID CATHODE, (2) MEANS INCLUDING A METAL COLLAR CONDUCTIVELY ENGAGING SAID ROD AND SUPPORTING SAID HEATER ON SAID ROD, SAID COLLAR HAVING OPPOSITE END FACES AND A SMALLER AXIAL DIMENSION THAN LATERAL DIMENSION, AND (3) AN INSULATING DISC SEATED ON SAID ONE OF SAID END FACES OF SAID COLLAR AND HAVING AN OPENING SNUGLY RECEIVING SAID ROD, SAID DISC EXTENDING TRANSVERSELY OF SAID CATHODE AND SNUGLY ENGAGING THE INTERNAL SURFACE OF SAID CATHODE, (4) SAID COLLAR HAVING A SMALLER OUTER DIAMETER THAN SAID DISC WHEREBY SAID DISC ELECTRICALLY INSULATES SAID COLLAR FROM SAID CATHODE. 